The present invention relates generally to printers, and, more specifically, to replaceable printer rolls therein.
A typical printer includes a roll of printing paper upon which any desirable information may be printed. The paper is wound in a continuous sheet on a supporting core, and the core is mounted on a driven spindle in the printer. In a thermal printer, the core includes thermal transfer ribbon wound thereon which is thermally activated during printing.
When the ribbon is depleted on the core, the empty core is removed from the spindle and replaced with a fully wound core for returning the printer to service.
The core typically includes retaining features for accurately retaining the core axially on the spindle in proper alignment with the printing mechanism, and circumferentially retaining the core around the spindle for rotating therewith as the spindle is driven during printer operation.
In one conventional design, the spindle includes three axial slots around the perimeter thereof which axially receive corresponding straight axial ribs projecting inwardly along the inner surface or bore of the core. The core may be easily inserted axially over the spindle by engaging the corresponding ribs and slots, with the ribs providing circumferential retention around the spindle for being driven in rotation therewith.
However, additional features are required for locking the core in axial position over the spindle and preventing its unintended liberation therefrom or misalignment thereon. This increases the complexity of the core and spindle assembly, and correspondingly increases the cost thereof.
Cost is a significant factor in the manufacture and use of printer rolls and must be minimized for maintaining competitive advantage in the market for supplying replacement printing rolls.
U.S. patent application Ser. No. 09/694,410, now U.S. Pat. No. 6,425,551 discloses a low cost core having three bowed ribs which frictionally engage the respective slots for retaining the core on the spindle. The primary core embodiment illustrated in FIGS. 1-5 of that application has been sold by the present assignee and in public use in this country for more than a year now.
The bow ribbed core is made of molded plastic, and experience in the field has now shown that random variations in rib dimensions due to the molding process exceeds the few mil tolerances of the drawing specification therefor and produces some cores with reduced frictional retention force from the out-of-spec bowed ribs. When such cores are used in a new printer having a horizontal spindle, the friction force is usually sufficient to retain the core on the spindle.
However, as the spindle wears during use it may become loose and can tilt downwardly a few degrees. That small tilt may then permit gravity to exceed the friction retention force of the ribs and allow the core to slide out of proper position on the spindle.
More expensive plastic may be used to accommodate increased bending loads in the ribs for increasing friction retention force, but that would reduce the competitive advantage of the product. And, increasing the bending loads leads to higher stress in the plastic ribs, and may cause premature fatigue failure of such bowed ribs.
Accordingly, it is desired to provide an improved core for winding sheet rolls thereon having corresponding retention features for being mounted to a supporting spindle.
A core includes a tubular body for supporting a wound sheet roll on a spindle. The body includes an annular outer surface for receiving the sheet roll, and an annular inner surface defining a bore for receiving the spindle. A plurality of ribs project inwardly from the body inner surface and extend axially between opposite first and second openings for nesting in the corresponding slots in the spindle. Each of the ribs includes a beveled fork for frictionally engaging a corresponding one of the spindle slots to frictionally retain the core axially thereon.